Internal combustion engines can generally be grouped into one of two classes, spark-ignited and compression-ignited engines. Spark-ignited engines typically operate by introducing a stoichiometric mixture of air and fuel into the cylinder of the engine. The piston then compresses this mixture, and at predetermined crankshaft angle, a spark plug will ignite the fuel and air mixture producing a flame front that propagates through the combustion chamber. The rapid increase in heat from the burned fuel triggers an increase in pressure which forces the piston downward in the cylinder. The ability to precisely time the combustion event through the use of a spark plug is a benefit of the spark-ignited engine. However, the spark-ignited engine may be somewhat inefficient since the compression ratio of the engine is kept to a lower than optimal level to avoid spark knock. Spark knock occurs when the fuel and air mixture ignites independently of the spark plug and may cause engine damage. Consequently, spark-ignited engines typically have compression ratios between 8 and 11.
The compression ignition engine, on the other hand, operates at relatively high compression ratio that is typically within the range of 15–22. This high compression ratio boosts the mechanical efficiency of the compression-ignited engine. The compression ignition engine operates by introducing unthrottled air into the cylinder, thereby increasing the efficiency over that of the throttled spark-ignited engine by decreasing pumping losses. In a traditional compression-ignited engine, the ignition timing is controlled by the injection of diesel fuel into the cylinder near the end of the compression stroke when the trapped air within the combustion chamber is of a sufficient temperature to ignite the fuel. The heat release of the combustion process causes an increase in in-cylinder pressure which then forces the piston downward in the same fashion as the spark-ignited engine.
The compression ignition engine, though efficient, can produce greater emissions than a spark-ignited engine. The two emission constituents that are of main concern are particulates or “soot” and oxides of nitrogen or NOx. Soot is formed in locally rich of stoichiometric areas within the combustion chamber. These overly rich areas occur due to the non-homogeneity of the fuel and air charge occasioned by the lack of adequate premixing of the fuel and air upon ignition. The formation of oxides of nitrogen is a function of combustion chemistry. The compression-ignited engine, due to its locally stoichiometric combustion, combusts the fuel and air mixture at high temperatures causing high NOx emissions in the exhaust stream. The expansion process occurs so quickly that the chemical reactions are frozen and the NOx quantities are unable to reach chemical equilibrium resulting in high engine-out NOx emissions. The treatment for NOx has traditionally been converter systems such as Selective Catalyst Reduction (SCR) for engines operating with lean of stoichiometric air and fuel ratios. The SCR system injects urea or ammonia into the exhaust stream to convert NOx into nitrogen and water. Engine designers may also seek to lower the temperature of combustion in a compression-ignited engine by decreasing the compression ratio. This may lessen NOx formation and may also lead to decreases in efficiency and difficulties starting at cold temperatures.
Recent interest in fuel efficiency and federally-mandated emission requirements has refocused efforts to develop highly efficient, low emission compression-ignited engines and modes of operation. Examples of such technologies are the Premixed Charge Compression Ignition (PCCI) and the Homogeneous Charge Compression Ignition (HCCI) modes of combustion. The PCCI mode of combustion incorporates a standard compression ignition combustion system with high rates of aggressively cooled exhaust gas recirculation (EGR) and an early start of injection (SOI) timing. The combination of high EGR rates and early SOI result in a long ignition delay period prior to the start of combustion (SOC). The ignition delay period exceeds the fuel injection duration during PCCI combustion resulting in a premixed combustion event at the SOC. The premixed nature of the fuel and air mixture, along with a high EGR rate, reduces the formation of locally rich, high temperature regions that contribute to soot formation. The high EGR rate acts as a charge diluent that suppresses the temperature of combustion below that which significant amounts of NOx are formed.
The PCCI combustion mode is effective at low engine speeds and loads where the amount of fuel injected is very low and the time available for premixing is very long. As the engine load increases, the amount of heat released in the rapid premixed burning process becomes large enough to create excessive combustion noise. This occurs even if there is sufficient premixing of the fuel and air during the ignition delay period. Excessive combustion noise is objectionable to consumers; consequently, PCCI combustion has a low engine load limit.
For a given noise constraint, HCCI combustion is capable of operating at higher engine loads than PCCI combustion since the nearly homogeneous fuel and air charge will result in lower localized temperature within the combustion chamber during combustion. Due to the low volatility inherent in diesel fuel, it is difficult to create a homogenous fuel and air mixture. To attempt such a mixture, early direct fuel injection must be used. A conventional diesel fuel injector has a wide injector spray cone, and the early injection timing required for HCCI combustion may cause significant fuel impingement on the cylinder wall. Fuel impingement on the cylinder wall inhibits the formation of a homogeneous fuel and air mixture and may cause fuel dilution of the lubricating oil. To correct these conditions, fuel injectors with different spray patterns and piston bowls to compliment the spray patterns are required. This is typically an expensive endeavor that requires much experimentation and development to perfect.
Premature ignition may occur with homogeneous fuel and air mixtures in compression-ignited engines. The high Cetane number of diesel fuel represents the relative ease in which the fuel and air mixture will ignite. This premature ignition can cause noise, poor performance, and engine damage in extreme cases. Therefore, to properly phase the SOC for an HCCI engine, a reduced compression ratio is necessary. This reduction in compression ratio will result in a decrease in efficiency. To maintain acceptable cold start performance, a high compression ratio must be maintained. Therefore, a variable compression ratio engine system is often employed in an HCCI engine.